How can I tell if my cornish chicken is a rooster or hen?

We got a chicken for free just as a biddie....not knowing what breed of chicken it was...We were thinking it was a white leghorn...so we within a month we purchased more......one definitely being a white leghorn rooster!! Well when we got them home we knew,the little chicken was not a leghorn and I did some research on what advice was given to me and we have a White Cornish Chicken!!! For some strange reason all the chickens kept pecking it, until we decided to keep them separated!!! Now that the Leghorn Rooster is older...he every time we let out the chickens to roam the yard freely....will go out of his way to attack my Cornish Chicken...my question is it maybe because my cornish chicken may be a rooster or what could it be? Help!!!!